A magnetostrictive torque sensor is suitably used for, for example, electric power steering (EPS) devices that transfer the power of an electric motor (hereinafter also simply referred to as a “motor”) to a steering system as an auxiliary steering force (a steering assist force) when a vehicle is steered using an operation member, such as a steering wheel in order to reduce the load imposed on a driver who operates the operating member (i.e., the steering load).
Such a magnetostrictive torque sensor used for the electric power steering device is described in, for example, Japanese Patent No. 4581002 (FIG. 2) filed by the present applicant. The magnetostrictive torque sensor employs NiFe plating as a magnetostrictive film.
As illustrated in FIG. 24, a magnetostrictive torque sensor 310 is formed by plating a Ni65[%]-Fe35[%] plating film (Ni Fe plating film) having a thickness of about 30 to about 40 μm on a sensor shaft 317, which is a shaft member formed by heat-processing a steel, such as S45C or SCM420, first. Thus, a magnetostrictive film mf is provided on the sensor shaft 317. Thereafter, a rotational torque is applied to the sensor shaft 317 in one direction, and the magnetostrictive film mf is heated by radio-frequency heating. After the magnetostrictive film mf is cooled, the rotational torque is removed so that anisotropy (magnetic anisotropy) ma1 is added. Subsequently, a rotational torque is applied in the opposite direction, and the other part of the sensor shaft 317 is heated by radio-frequency heating. After the part of the sensor shaft 317 is cooled, the rotational torque is removed so that anisotropy ma2 in a different direction is added.
Subsequently, a first detection coil 341 and a second detection coil 342 are wound around part of the magnetostrictive film mf having the anisotropy ma1 added thereto, and a third detection coil 343 and a fourth detection coil 344 are wound around part of the magnetostrictive film mf having the anisotropy ma2 added thereto.
The first detection coil 341 to the fourth detection coil 344 are connected to an interface circuit 320 serving as a torque detection circuit. The first detection coil 341 and the third detection coil 343 are connected in series. The first detection coil 341 is connected to a direct current voltage supply Vcc (the voltage is also referred to as “Vcc”) via a damping resistor Rd (the resistance value is also referred to as “Rd”). The third detection coil 343 is connected to a switching element 334 via a damping resistor Re (the resistance value is also referred to as “Re”). In addition, a free wheel diode 336 is provided. The free wheel diode 336 has an anode terminal connected to a connection point between the damping resistor Re and the switching element 334 and a cathode terminal connected to the direct current voltage supply Vcc.
An alternating current is applied to the first detection coil 341 and the third detection coil 343 by driving the switching element 334 using a square wave signal PVF. In such a case, by applying a steering torque from a steering wheel (not illustrated) to the sensor shaft 317, the magnetic permeability of each of the portions of the magnetostrictive film mf having the anisotropy ma1 and the anisotropy ma2 added thereto varies and, thus, the inductance of each of the first detection coil 341 and the third detection coil 343 varies. For example, the inductance of the first detection coil 341 increases, whereas the inductance of the third detection coil 343 decreases.
At that time, the same alternating current flows through the first detection coil 341 and the third detection coil 343. Accordingly, a divided voltage V1 generated by a voltage caused by the product of the inductance of the first detection coil 341 and an electric current variation and a voltage caused by the product of the inductance of the third detection coil 343 and the electric current variation appears at a middle point 351 between the first detection coil 341 and the third detection coil 343 (the divided voltage V1 is also referred to as a “middle point voltage”). The middle point voltage V1 of the middle point between the first detection coil 341 and the third detection coil 343 is held by a bottom hold circuit 331 and is amplified. In this manner, an output voltage VT1 can be obtained.
Similarly, the second detection coil 342 and the fourth detection coil 344 are connected in series. The fourth detection coil 344 is connected to the direct current voltage supply Vcc via the damping resistor Rd, and the second detection coil 342 is connected to the switching element 334 via the damping resistor Re. An alternating current is applied to the second detection coil 342 and the fourth detection coil 344 by driving the switching element 334 using the square wave signal PVF. In such a case, by applying the steering torque from the steering wheel (not illustrated) to the sensor shaft 317, the magnetic permeability of the portions of the magnetostrictive film mf having the anisotropy ma1 and the anisotropy ma2 added thereto varies and, thus, the inductance of each of the second detection coil 342 and the fourth detection coil 344 varies. For example, the inductance of the second detection coil 342 increases, whereas the inductance of the fourth detection coil 344 decreases.
At that time, the same alternating current flows through the second detection coil 342 and the fourth detection coil 344. Accordingly, a divided voltage V2 (a middle point voltage V2) generated by a voltage caused by the product of the inductance of the fourth detection coil 344 and an electric current variation and a voltage caused by the product of the inductance of the second detection coil 342 and the electric current variation appears at a middle point 352 between the second detection coil 342 and the fourth detection coil 344. The middle point voltage V2 of the middle point between the second detection coil 342 and the fourth detection coil 344 is held by a bottom hold circuit 332 and is amplified. In this manner, an output voltage VT2 can be obtained.
In addition, an output voltage, which is a differential output of a differential amplifier 333 generated from the output voltage VT1 and the output voltage VT2, is obtained as a steering torque signal VT3. Thereafter, a target electric current to be applied to a steering assist motor (not illustrated) is calculated on the basis of the steering torque signal VT3.
Furthermore, malfunction is detected using the output voltage VT1 and the output voltage VT2. More specifically, if the sum voltage of the output voltage VT1 and the output voltage VT2 is outside a predetermined voltage range, it is determined that the magnetostrictive torque sensor 310 malfunctions.